1. Field of the Invention
The present invention relates to a varistor body and a varistor having the same.
2. Related Background Art
Varistor is an element showing a voltage nonlinear characteristic which keeps insulation property with high resistance up to a certain voltage, and suddenly becomes a low resistance above that voltage to allow current to flow therethrough, (hereinafter referred to as the “varistor characteristic”). Utilizing the characteristic, the varistor is used as an element and the like for protecting circuit of electronics devices in case of generation of abnormal voltage (surge). In recent years, the varistor has become miniature scale, and is expected to be used as an inexpensive surge-protection element in digital cameras, mobile phones, and the like substituting for existing Zener diode.
Known varistors include a laminated type varistor having a structure of a varistor body prepared by laminating alternately a varistor layer providing the varistor characteristic and an internal electrode layer, and an external electrode which is attached to outside of the varistor body and is connected to the internal electrode layer. Other than that type, there is known an array type varistor having a structure of combining pluralities of above elements.
The varistors having above structures are often fixed and connected to printed circuit board and the like by soldering the external electrode. Ordinary external electrodes are, however, likely fused and dispersed into the solder, which likely induces connection failure. To cope with the drawback, conventional external electrodes are prepared with a substrate electrode and a plating layer of Ni and the like formed on the surface of the substrate electrode, thus improving the heat resistance. From the point of production cost and other variables, the formation of that type of plating layer is generally done by electroplating.
Since, however, the varistor body (varistor layer) has above-described semiconductor characteristic, the insulation resistance is inherently not so large. As a result, on applying electroplating, there are conventionally often occurred the formation of plating with running-over portion thereof from the range to form the substrate electrode, (that kind of phenomenon is hereinafter referred to as the “plating extension”), and the adhesion of plating to positions other than the substrate electrode, (that kind of phenomenon is hereinafter referred to as the “plating adhesion”). Those phenomena of plating extension and plating adhesion are not welcomed because they have become significant causes of short-circuit between external electrodes along with the recent movement of miniaturization of varistor.
To avoid the problem, there is a known countermeasure which covers the surface of varistor body, other than the external electrode, by an insulation coating layer such as glass coat. That type of countermeasure, however, requires forming the glass coat film at precise dimensions, which raises other problems such as complex production process and increased production cost.
Another countermeasure to attain good electroplating is disclosed as a method of diffusing Li or Na into a region in the vicinity of surface of the varistor body, (refer to Japanese Patent Application Laid-Open No. 9-246017). The method brings the region in the vicinity of the surface of varistor body to high resistance, thus suppressing the plating formation on other part than the substrate electrode.